Voltage controlled oscillators are well known devices used in a wide variety of RF electronics applications such as frequency synthesizers of RF communications systems. Despite recent advances in their designs, VCOs are still considered one of the most critical design components in RF transceivers. In general, the most important parameters of a VCO are phase noise, power consumption and frequency tuning range. An output buffer circuit is often used to amplify the output of the VCO and to isolate the VCO from load conditions.
In order to meet stringent phase noise specifications of 3G wireless communications standards such those of the CDMA 1X and other protocols, current VCOs produce a differential output voltage with a wide swing, typically up to 3V. This wide voltage swing tends to diminish reliability of CMOS transistors within buffer circuits buffering the VCO output. The wide swing applied to the buffer results in higher than desirable gate-to-drain and gate-to-source voltages VGD and VGS across the buffer circuit transistors, thereby stressing those transistors and creating reliability issues due to both hot carrier injection (HCI) and gate oxide breakdown. Reliable operation is of paramount importance and becomes more challenging as deep submicron processes are used.
Moreover, in portable wireless devices it is beneficial to keep power consumption to a minimum to prolong battery life. Realizing phase noise performance, and in particular, far out phase noise, is important as SAW filters are eliminated from transceiver integrated circuits.
It is desirable to achieve low current consumption and high reliability goals for a VCO and its buffer circuit while maintaining low phase noise operation.